The invention relates in general to generator systems for motor vehicles and, in particular, to a generator system for a motor vehicle comprising an asynchronous generator and a converter, as well as a process for the activation of the converter.
Because of progress in the area of power electronics, rotating field drive units supplied with power by converters have become more widespread, such as in machine tool-making machines and in rail vehicles, for example, over the course of the past 25 years. For the operation of a rotating field motor, it is generally necessary to provide an alternating current system in the machine, the frequency of which system is changeable. So-called converters, which are able to generate alternating currents of any frequency desired, mostly on the basis of an alternating current source of set frequency, such as 50 Hz power line frequency, through a direct current power supply network, for example, are able to do this, A converter consequently comprises a rectifier, which converts an alternating current into a direct current, and a converter, which in turn produces an alternating current or an alternating current of variable frequency, as the case may be, from a direct current.
Converters which carry out a sinus-evaluated pulse width modulation and which therefore produce rectangular pulses, for example, which yield, on average, an approximately sinusoidal voltage curve, are usually used. In an earlier publication (J. Schxc3x6rner, Reference voltage for converter control in: ETZ-B, Volume 27, 1975, page 7), it was proposed, in an asynchronous drive unit, to superimpose a third harmonic oscillation on the sinusoidal voltage curve, or else to select a rectangular voltage curve. This should serve for the purpose, on the one hand, of exposing the semi-conductor switch in the converter that is sensitive to the voltage of that time to only aslittle voltage as possible but, on the other hand, to make an effective voltage amplitude that is as high as possible available for the operation of the motor. A similar point is known from R. Bohnert, Ru-Song Wu, Improved three-phase pulse Grant, The use of harmonic distortion to increase the output voltage of a three-phase PWM inverter in IEEE Trans. Int. Appl., Volume 1A-20, No. 5, 1984.
It was proposed at an earlier date to use rotating field machines in synchronous- or asynchronous construction as combined generators and starters in the motor vehicle and, specifically so, particularly as so-called xe2x80x9ccrankshaft starting generatorsxe2x80x9d (see WO 97/08456, for example). These proposals are connected with the awareness that considerable additional savings in fuel consumption are possible through the fact that mechanically-operated auxiliary aggregates, such as internal combustion engine valve controls, air conditioner compressors, and servomechanisms for steering and braking, are switched over to an electrical drive. The novel motor vehicle generators proposed can provide the electrical performance that is thereby necessary over the great range of speed that arises, and with a high level of efficiency.
The invention provides, in accordance with a first aspect, a generator system, particularly for a motor vehicle, which comprises an asynchronous generator and a converter. The converter serves for the production of converter output voltages from a direct current source which form a three-phase system, which produce, for the formation of rotating fields that are suitable for the generator function, a rotating field in the machine that is suitable for the generator function. The converter output voltages have a frequencyxe2x80x94referred to in the following as the xe2x80x9cCO frequencyxe2x80x9dxe2x80x94that can be changed in dependence on the range of the generator speed. The voltage curve of the converter output voltages can be broken down into a primary component with a sinusoidal voltage curve of the CO frequency, as well as into at least one additional secondary component with a sinusoidal voltage curve with an odd-numbered multiple of the CO frequency. The components are thereby weighted in such a manner that the amplitude of the primary component is greater than the amplitude of the voltage curve of the converter output voltages, so that a voltage magnification is brought about through this.
In accordance with one additional aspect, the invention is directed to a method for the activation of a converter of a synchronous generator, particularly for a motor vehicle. It serves for the production of converter output voltages from a direct current source, which form a three-phase system and which produce a rotating field for the generator function in the machine that is suitable for the formation of rotating fields suitable for the generator function. The converter output voltages have a frequency (CO frequency) that can be changed in dependence on the rotational speed of the generator. The voltage curve of the converter output voltages can be broken down into a primary component with a sinusoidal voltage curve of the CO frequency, as well as into at least one additional secondary component with a sinusoidal voltage curve with an odd-numbered multiple of the CO frequency. The components are thereby weighted in such a manner that the amplitude of the primary component is greater than the amplitude of the voltage curve of the converter output voltages, so that a voltage magnification is brought about by this means.